The Joint Venture of CMS-ITECO-FBC-BDA successfully completed the consulting services for the construction supervision of Headworks of Sikta Irrigation Project.
Headworks of Sikta Irrigation Project
The objective of the Sikta Irrigation Project (SIP) is to provide year-round (YR) irrigation to a cultivable command area of 33,766 ha on the right bank of the Rapti River, including the rehabilitation of Dunduwa Irrigation System built in 1964 under the Indian Cooperation Mission. It also envisages improving the intake system of the existing Rajkulo Irrigation Scheme, which is currently providing irrigation to around 1,800 ha on the left bank of the West Rapti River, and extending the area irrigated on the left bank of the Rapti River to about 9,000 ha, including the area of 2,000 ha covered by the existing Fattehpur Irrigation Scheme. The West Rapti River is the only viable surface source to provide irrigation to these areas. The right bank command area lies in 34 different Village Development Committees (VDC) and Nepalgunj Municipality of Banke District.
The concept of providing YR irrigation using the Rapti River as the source to optimize agricultural production in Banke district dates back to 1975, when a pre-feasibility study was conducted for the West Rapti Multipurpose Development Project. The study recommended construction of a storage project with electricity generation and irrigation as the main objectives. Following this, in August 1980, Lahmeyer International GmbH conducted the feasibility study of the Sikta Irrigation Project. The study recommended a run-of-river gravity irrigation system.
A separate feasibility study was completed in June 1983 by the Department of Irrigation, Hydrology and Meteorology, which recommended a run-of-the-river gravity irrigation system by constructing a 433 m long barrage across the Rapti River.
The most recent study of the Project was undertaken by the Department of Irrigation (DOI) in 2002 under the Irrigation Development Programme – Mid-western Region (IDP-MWR), funded by the European Union (EU). A team of in-house engineers designed the headworks and associated components of the Project. After various deliberations with the experts and the key personnel of the DOI the following key design parameters were adopted:
- A barrage type diversion structure for 100-year return period flood as the design flood;
- A settling basin after the intake structure on the right bank to exclude most of the suspended sediment load;
- A single lane road bridge over the barrage;
- Provision of fish pass; and
- Rehabilitation or integration of the existing Rajkulo Scheme as appropriate.
Implementation of the Sikta Irrigation Project commenced in July 2006, with the signing of the construction contract for the Headworks (Contract No: SIP/HW/ICB/01) with SINOHYDRO-LUMBINI JV and the construction supervision contract with the Joint Venture of Consolidated Management Services Nepal (P) Ltd. (CMS) in the lead, ITECO Nepal (P) Ltd. (ITECO), Full Bright Consultancy (P) Ltd. (FBC) and BDA (P) Ltd. (BDA).
The civil works of the Sikta Headworks has been completed, but the gates of the barrage are yet to be installed, for which the Consultant (CMS-ITECO-FBC-BDA JV) has already completed the design. The major components that make up the Headworks are:
1. The barrage: The structure constructed across the Rapti River is the main diversion structure that will raise the river's water level to the desired head and control the flow. The barrage is 317 m long, with 13 spillway bays that will require 17 m wide by 4.0 m high gates and five undersluice bays that will require 10 m wide and 5.5 m high gates. Two undersluices with total spans (including intermediate piers) of 22.0 m and 34.0 m are provided in the left and right ends of the barrage respectively to ensure relatively clear and smooth flow of water towards the left as well as right intakes. A single lane road bridge is provided over the barrage.
2. Intake: In this system, there are two intakes – on the left and right of the diversion barrage. The right bank intake structure is designed for a discharge withdrawing capacity of 62.5 m3/s at Pond Level. The crest of the intake is placed 3.5 m above the undersluice crest level. There are, in total, 14 bell mouthed entrances that converge to 7 barrels. The gates are placed at the end of the barrels. The gates have been motorized.
The left bank intake is designed for 14.7 m3/s capacity. The intake crest position, orientation and the design of entrance are similar to that of the right intake. There are two gates in the left intake. The left intake gates have not been motorized and for the time being, these gates will have to be operated manually.
3. Sediment removal arrangement: A 610 m long settling basin is provided downstream of the right bank intake to maintain the sediment size and concentration to the acceptable level of the Main Canal by trapping excess sediment load.
The settled sediment in the basin has been planned to be flushed and returned to the Rapti river again using hydraulic flushing arrangement. The bed width of the basin is 50 m, with two separator walls that extend up to the basin length. Near the entrance of the basin, submerged vanes have been provided to guide the flow and the sediment equally to the whole width of the basin.
4. Gates and Stoplogs: Gates are used to control flow through the main diversion structure as well as through the intakes. Stop logs are used for emergency closure of the flow through the bays of a barrage or an intake. Gates and Stoplogs for the barrage are yet to be installed.
5. River training works: These arrangements are necessary to guide the water towards the barrage and from the barrage towards the natural river course without causing any harm to the Headworks. The river training works for the Headworks have already been completed.
6. Fish pass: One of the major environmental concerns of a dam, barrage or weir across a perennial river is that it acts as a barrier for free movement up and down of aquatic life including migratory fishes. This concern has been addressed by providing a fish pass adjacent to the right under-sluice. The fish pass consists of a series of successive pools with small drops in its water surface at the cross walls.
The fish pass provided in Sikta Barrage has slightly flatter slope as compared other barrages in Nepal, so that it will safely perform its intended function, which is to facilitate the migratory fish to swim upstream during summer and monsoon for hatching and in search of colder temperature.